Technical Field
Embodiments of the subject matter disclosed herein generally relate to marine seismic sources configured to emit waves at different predetermined depths selected by taking into consideration the effect of the water-surface reflections of the waves (known as “ghosts”).
Discussion of the Background
Seismic surveys are used to probe an underground structure to obtain an astute indication about the hydrocarbon content thereof. During such a seismic survey, seismic waves, which penetrate the underground structure, bounce off its layer interfaces and are detected by seismic receivers. Seismic survey methods are used in land and in marine environments.
In a marine environment, seismic waves are emitted by a marine seismic source and detected by seismic receivers (hydrophones, marine particle motion sensors such as accelerometers or differential pressure sensors, geophones, or any other type of acoustic or elastic wave detector). The source and the receivers may be towed over the surveyed structure (which is under the seafloor) or placed on the seafloor. The source often includes plural individual devices such as air guns, pingers, sparkers, boomers, marine vibrators, other acoustic emission devices, or combinations thereof. The source may emit energy (i.e., waves) impulsively, continuously, or semi-continuously, with a predetermined signature (mono-frequency, pseudo-random sweep, regular sweep, etc.).
A seismic source emits waves (i.e., energy) in all directions away from the source. Up-going waves, which propagate toward the water surface, are reflected at the water-air interface. The water-surface reflected waves, known as “ghosts,” have opposite polarity than the up-going waves and propagate toward the seafloor. Ghosts interfere with the energy which left the source downwards. The energy which left the source downwards is often termed ‘primary energy’. The energy reflected at the sea surface is termed ‘ghost energy’.
For conventional sources, the presence of ghosts increases the complexity of seismic data processing by causing signals with complex source signatures (i.e., amplitude versus time at large distance from the source) incident to the seafloor. The complex signature signal is a challenge for data processing and limits data density. Methods for de-ghosting seismic data as described, for example, in U.S. Patent Application Publication 2013/0279290 are challenging. It would be desirable to improve the seismic sources such that to eliminate the complexity due to the ghosts at data acquisition stage, and thus to simplify the seismic data processing.